ABSTRACT Size-Resolved Measurement of Ultrafine and Nanometer Particle Concentrations This proposal addresses the need to assess worker and community exposure to engineered nanomaterials and other ultrafine particles. The ultimate aim is the development of a compact, battery powered instrument for size- and time-resolved measurement of ultrafine and nanometer particle number concentrations. To reach this goal, two novel technologies are used: (1) the opposed migration aerosol classifier developed at the California Institute of Technology, and (2) a self-sustaining laminar-flow water condensation particle counter being developed by our firm. Like the commonly-used scanning mobility particle size spectrometers, the opposed migration aerosol classifier uses electrical mobility classification, but its cross-flow geometry provides an inherently more compact instrument. The self-sustaining laminar-flow water condensation counter that serves as the detector has the same wide dynamic range of commercial counters, but does not require liquid fill reservoirs, and can be operated in any orientation. In contrast to the existing methods, our approach is inherently smaller and lighter. This Phase I project aims to show that it can be made to be unobtrusive, battery operable and suitable for personal monitoring. Specific objectives for this Phase I project are: 1) to demonstrate electrical mobility particle size classification spanning from 10 nm to 200 nm, with a resolution of 6 or more channels in less than 60s; 2) to demonstrate water-based condensation particle detection with at least 90% detection at 10 nm in an instrument that may be tipped, and that sustains 24-hr operation without the addition of water, 3) to construct an integrated mobility separator and detector module that is no more than 15 cm in any dimension; 4) to evaluate the accuracy, size resolution, and time response through tests with monodisperse aerosols and comparison under ambient conditions to a standard scanning mobility particle size spectrometer; and 5) to evaluate the suitability to be packaged as a wearable particle-sizing detector based on measured performance, size, weight, and power of the sizing classification and detector module.